The present invention relates to a method for measuring the surface state such as structure, form or the like of an object by using one of a variety of measuring apparatuses such as interferometer, moire fringe measuring instrument, etc. and more particularly to a method for divisional measurement of an object surface in which its surface is imaginarily divided into a plurality of fractions in case its whole surface area is wider than the measuring aperture of a measuring apparatus used and each of the surface fractions is measured.
Also, the present invention relates to a method for contiguously joining together the plurality of object surface fractions of an object after subjected to the divisional measurement in case the object has a whole surface area wider than the measuring aperture of a surface state measuring apparatus used.
For highly accurate measurement of the state of a plain or curved surface (referred to hereinafter as "object surface") of various kinds of parts (hereinafter referred to as "object"), a variety of apparatuses (hereinafter referred to as "form measuring apparatus") using an interferometer or the like has been proposed which can determine the surface state of the object from the pattern of an interference fringe or the like formed by the measuring apparatus during the measurement. However, the area the form measuring apparatus can cover by one measurement is limited and so an object surface wider than the measuring aperture of the apparatus must be subjected to a plurality of measurements.
A solution has been proposed as known from the disclosure in Unexamined Japanese Patent Publication Nos. Hei 2-259509 and 2-259510. This solution is such that when measuring a large object such as seen in FIG. 1 having a wide surface, the object surface is imaginarily divided into a plurality of fractions (hereinafter referred to as "sub-areas") and each of these sub-areas is measured. Namely, this method permits to provide an image of the whole surface of a large object, by reconstructing from the data obtained by the measurement of an image (hereinafter referred to as "reconstructed image") representing the state of each sub-area, and joining together and integrating the reconstructed images.
The method for such divisional measurement can be used to obtain accurate measured data of each of the sub-areas. However, simple joining of the sub-area images reconstructed from the measured data is not sufficient for determining the state of the object surface but the sub-areas are discontinuous at their junctions in many cases so that it is difficult to obtain an accurate image of the whole object surface corresponding to a real state. In addition, when the actual measurement is repeatedly done of the sub-areas, the shift of the measurement from one of the sub-areas to another will possibly cause (1) minute vertical oscillation of the object surface; (2) failure in recognition of the sub-area currently under measurement; (3) large error in boundary between sub-areas; and (4) missing of measurement during repetition of the measurement.
When, for example, a glass plate of 1 cm in thickness is measured by repeating the measurement for each of the sub-areas of this object, a temperature change as small as 1.degree. C. of the surface will result in an expansion or shrinkage of the order of 0.1 .mu.m since the glass plate has a coefficient of linear thermal expansion of 1.times.10.sup.-5 [deg.sup.-1 ]. Thus, it is essential that the measurement should be done as efficiently and rapidly as possible. More particularly, the testing person's temperature will possibly have a delicate influence causing a non-negligible change in expansion of the object surface.